Bias control of optical modulators

ABSTRACT

An optical waveguide modulator with automatic bias control is disclosed. A portion of the modulator light is mixed with reference light and converted to one or more electrical feedback signals. An electrical feedback circuit controls the modulator bias responsive to the feedback signals.

FIELD OF THE INVENTION

The invention generally relates to photonic integrated circuits, andmore particularly relates to an apparatus and method for an automatedbias monitoring and control of an optical quadrature modulator.

BACKGROUND OF THE INVENTION

Optical waveguide modulators used in high-speed optical communications,such as those based on waveguide Mach-Zehnder (MZ) interferometricstructures, may require active control of their operating conditions,and in particular of their bias voltage that sets the relative phase ofinterfering light waves in the modulator in the absence of themodulation signal. The waveguides of the modulator are typically formedin an electro-optic material, for example a suitable semiconductor orLiNbOx, where optical properties of the waveguide may be controlled byapplying a voltage. Such a waveguide modulator may be a part of anoptical integrated circuit (PIC) implemented in an opto-electronic chip.

Very high speed optical systems may benefit from Quadrature AmplitudeModulation (QAM), which may be realized using a quadrature modulator(QM) that may be implemented using nested MZ interferometric structures.Such structures typically require controlling several bias voltages. Forexample, a QAM optical signal may be generated by splitting light from asuitable light source between two MZ modulators (MZM) driven by anin-phase (I) and a quadrature (Q) complements of an electrical QAMsignal carrying data, and then combining the resulting I and Q modulatedlight signals in quadrature, i.e., with a 90°, or π/2 radians (rad),relative phase shift ϕ_(IQ). For example the two MZMs of such QM mayeach be modulated by a BPSK (binary phase shift keying) signal whilebeing biased at their respective null transmission points for push-pullmodulation. When their outputs are added together in quadrature, i.e.with the relative phase shift ϕ_(IQ)=π/2, a QPSK signal (Quaternaryphase shift keying) results. While the bias voltages of the two MZMz forthe push-pull modulations may be controlled by monitoring thetime-averaged optical power at the output of the modulator, the outputaveraged optical power is insensitive to the IQ phase shift ϕ_(IQ), sothat a drift of the bias voltage V_(IQ) away from a bias point needed tomaintain the desired IQ phase shift may be more difficult to monitor andcorrect for. Known techniques for monitoring the IQ phase shift ϕ_(IQ)in the modulator typically require high-bandwidth processing of thecontrol signal, which is difficult to implement in practice.

Furthermore, existing feedback schemes that are used to control a setpoint of an optical MZ modulator typically require tapping off a smallportion of the modulator output power to analyze for bias drifts. Thetapped-off portion of the output power, although relatively small,should still be large enough in the conventional bias control techniquesso that relatively small bias drifts may still be detected, which maymeasurably reduce the useful optical power from the modulator.

Accordingly, it may be understood that there may be significant problemsand shortcomings associated with current solutions and technologies forcontrolling a bias point of an optical waveguide modulator suitable foruse in high-speed optical systems.

SUMMARY OF THE INVENTION

Accordingly, one aspect of the present disclosure relates to an opticalmodulator device comprising:

(a) an optical modulator circuit (OMC) configured to modulate signallight at a target data rate and to produce modulated light, the OMCcomprising a bias electrode configured to receive an electrical biassignal for controlling a modulator set point, and an optical tap portconfigured to provide tapped light indicative of the modulator setpoint;

(b) an optical mixer (OM) comprising a first optical port opticallyconnected to the optical tap port of the OMC for receiving the tappedlight and a second optical port for receiving reference light, theoptical mixer configured to mix the reference light with the tappedlight and to produce one or more mixed light signals each combining thereference and tapped light; and

(c) a photodetector (PD) circuit comprising one or more photodetectors(PDs) and configured to convert the one or more mixed light signals intoone or more electrical feedback signals responsive to changes in themodulator set point.

One aspect of the present disclosure provides an optical waveguidemodulator system comprising an optical waveguide modulator comprising a)an optical modulator circuit (OMC) configured to modulate signal lightat a target data rate and to produce modulated light, the OMC comprisinga bias electrode configured to receive an electrical bias signal forcontrolling a modulator set point, and an optical tap port configured toprovide tapped light indicative of the modulator set point, b) anoptical mixer (OM) comprising a first optical port optically connectedto the optical tap port of the OMC for receiving the tapped light and asecond optical port for receiving reference light, the optical mixerconfigured to mix the reference light with the tapped light and toproduce one or more mixed light signals each combining the reference andtapped light; and c) a photodetector (PD) circuit comprising one or morephotodetectors (PDs) and configured to convert the one or more mixedlight signals into one or more electrical feedback signals responsive tochanges in the modulator set point, the optical waveguide modulatorsystem further including an electrical feedback circuit (EFC) connectingthe PD circuit with the bias electrode and configured to generate theelectrical bias signal in dependence on the one or more electricalfeedback signals.

An aspect of the present disclosure provides a method to operate anoptical modulator circuit (OMC) comprising an input port for receivingsignal light, an output port for transmitting modulated light, a biascontrol port for receiving an electrical bias signal controlling amodulator set point, and a tap port for providing tapped lightindicative of the modulator set point, the method comprising: a) mixingthe tapped light with reference light of a greater power in an opticalmixer to obtain one or more mixed light signals wherein the tapped lightis combined with the reference light; and, b) using a PD circuitcomprising one or more PDs to convert the one or more mixed lightsignals into one or more electrical feedback signals comprisinginformation about the modulator bias.

In accordance with an aspect of the present disclosure, the methodfurther includes c) generating the electrical bias signal in dependenceon the one or more electrical feedback signals so as to maintain themodulator bias at a desired set point.

In accordance with one aspect of the disclosure, the method may beapplied to the OMC that comprises a quadrature modulator configured tocombine two modulated optical signals in quadrature, the quadraturemodulator comprising a first optical phase shifter electrically coupledto the bias control port for varying an optical phase shift between thetwo modulated optical signals for setting the modulator bias. Step (a)of the method may then comprise obtaining first and second mixed lightsignals wherein the tapped light is added to the reference light with aphase shift that differs by 180□ between the first and second mixedsignals, and step b) comprises differentially detecting the first andsecond mixed light signals to obtain a first differential PD signal, andrectifying the first differential PD signal to obtain the firstelectrical feedback signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments disclosed herein will be described in greater detail withreference to the accompanying drawings, which may be not to scale and inwhich like elements are indicated with like reference numerals, andwherein:

FIG. 1 is a schematic diagram of an optical waveguide modulator systemhaving an optoelectronic feedback look for automatic control of themodulator bias incorporating an optical mixer for combining tapped-offmodulator light with reference light.

FIG. 2A is a schematic diagram of an input optical circuit for theoptical waveguide modulator system of FIG. 1 wherein the modulator lightand the reference light originate from a same optical source.

FIG. 2B is a schematic diagram of an input optical arrangement for anembodiment of the optical waveguide modulator system of FIG. 1 whereinthe modulator light and the reference light originate from differentoptical sources.

FIG. 3 is a general flowchart of a method for an automatic control ofthe modulator bias.

FIG. 4A is a schematic diagram illustrating an example packagingarrangement of the modulator system of FIG. 1.

FIG. 4B is a schematic diagram illustrating another example packagingarrangement of the modulator system of FIG. 1.

FIG. 5 is a schematic diagram of a waveguide quadrature modulatorincorporating a tunable optical phase shifter for controlling a relativeoptical phase between in-phase (I) and quadrature (Q) optical signals.

FIG. 6 is a schematic diagram of an embodiment of the waveguidequadrature modulator of FIG. 5 in the form of a nested Mach-Zehndermodulator.

FIG. 7 is a schematic diagram of an example optical mixing receivercircuit that may be used in the bias control loop of the modulatorsystem of FIG. 1, wherein a 90° optical hybrid is followed by twobalanced photodetector (PD) circuits.

FIG. 8 is a schematic diagram of one embodiment of the optical mixingreceiver circuit of FIG. 7 illustrating an example implementation of thebalanced photodetector (PD) circuits.

FIG. 9 is a schematic diagram of an embodiment of the optical mixingreceiver circuit of FIG. 7 having two RF rectifying circuits at theoutput.

FIG. 10 is a schematic diagram of an embodiment of the modulator systemof FIG. 1 including a quadrature modulator and an optoelectronicfeedback for controlling an IQ bias of the quadrature modulator.

FIG. 11 is a schematic graphical representation of a distorted BPSKconstellation with a non-ideal IQ phase shift.

FIG. 12 is a flowchart of a method for controlling the IQ bias of thequadrature modulator system of FIG. 10.

FIG. 13 is a schematic block diagram of an embodiment of an electricalfeedback circuit (EFC) of the quadrature modulator system of FIG. 10configured to equalize rectified balanced PD signals from the output ofthe optical mixing receiver circuit of FIG. 9.

FIG. 14A is a schematic block diagram of a first example embodiment ofthe EFC of the quadrature modulator system of FIG. 10 configured tocontrol the IQ bias of the quadrature modulator based on the averaged RFpower of a balanced PD signal from the optical mixing receiver circuit.

FIG. 14B is a schematic block diagrams of a second example embodiment ofthe EFC of the quadrature modulator system of FIG. 10 configured tocontrol the IQ bias of the quadrature modulator based on the averaged RFpower of a balanced PD signal from the optical mixing receiver circuit.

FIG. 15 is a schematic block diagram of an optical mixing receivercircuit including a 180° 2×2 optical mixer followed by a balanced PDcircuit and a line-rate RF power detector.

FIG. 16 is a schematic diagram of an embodiment of the optical mixingreceiver circuit of FIG. 9 with a summing circuits at the output.

FIG. 17 is a schematic diagram of an example implementation of thewaveguide modulator system of FIG. 1 or 10.

FIG. 18A is a schematic circuit diagram of one example embodiment of anRF rectifier.

FIG. 18B is a schematic circuit diagram of another example embodiment ofan RF rectifier.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular opticalcircuits, circuit components, techniques, etc. in order to provide athorough understanding of the present invention. However, it will beapparent to one skilled in the art that the present invention may bepracticed in other embodiments that depart from these specific details.In other instances, detailed descriptions of well-known methods,devices, and circuits are omitted so as not to obscure the descriptionof the present invention. All statements herein reciting principles,aspects, and embodiments of the invention, as well as specific examplesthereof, are intended to encompass both structural and functionalequivalents thereof. Additionally, it is intended that such equivalentsinclude both currently known equivalents as well as equivalentsdeveloped in the future, i.e., any elements developed that perform thesame function, regardless of structure.

Furthermore, the following abbreviations and acronyms may be used in thepresent document:

-   -   CMOS Complementary Metal-Oxide-Semiconductor    -   GaAs Gallium Arsenide    -   InP Indium Phosphide    -   LiNO₃ Lithium Niobate    -   PIC Photonic Integrated Circuits    -   SOI Silicon on Insulator    -   PSK Phase Shift Keying    -   BPSK Binary Phase Shift Keying    -   QAM Quadrature Amplitude Modulation    -   QPSK Quaternary Phase Shift Keying    -   RF Radio Frequency

Note that as used herein, the terms “first,” “second” and so forth arenot intended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. The word‘using’, when used in a description of a method or process performed byan optical device such as a polarizer or a waveguide, is to beunderstood as referring to an action performed by the optical deviceitself or by a component thereof rather than by an external agent.

The term “180° optical mixer” refers to an optical device that combinestwo input optical signals to produce two mixed optical signals whereinone of the two input optical signals is added to another of the twoinput optical signals with a phase offset of 180°, or in radian,therebetween. It will be appreciated that a 90° optical hybrid (OH) thatis conventionally used in coherent optical detection schemes may beviewed as an example of a 180° optical mixer (OM) that produces twopairs of such counter-phase mixed optical signals with a 90° shift inthe input signals phase offsets therebetween.

One aspect of the present disclosure relates to an optical waveguidemodulator which must be suitably biased, or kept at a desired set pointof its transfer characteristic, to have a desired modulationcharacteristic. The electrical signal that is required to maintain thedesired modulator bias or set-point may be referred to herein as thebias electrical signal, and may be typically but not necessarilyexclusively, in the form of dc bias voltage, which may be denoted Vb. Inoperation the modulator may experience changes in some of itsproperties, for example due to changes in its temperature or due tointernal to modulator processes such as aging or impurity drift, whichmay cause the bias voltage that is required to maintain the desired setpoint to drift, resulting in a deterioration of one or more aspects ofthe modulator performance, and therefore necessitating a way to monitorthat drift and to adjust the bias voltage accordingly. One way toaccomplish that is to monitor the output optical signal from themodulator to detect the drift.

With reference to FIG. 1, there is schematically illustrated an examplemodulator device 100 which is configured to be operated in an automaticbias control scheme. It includes an optical modulator circuit (OMC) 110which has an input optical port 111, an electrical bias control port114, a main output optical port 117, and a tap optical port 118. Themodulator device 100 further includes an optical mixing receiver (OMR)130 that is optically coupled to the tap port 118 and is configured toprovide, at its electrical port 139, one or more electrical feedbacksignals that are indicative of the modulator bias or the set point atwhich the modulator operates. An electrical feedback circuit (EFC) 180may be connected between the electrical port of the OMR 130 and the biascontrol port 114 of the OMC 110 to close the bias control feedback path.

The OMC 110 may conveniently be embodied using optical waveguides formedin or upon a support substrate of an electro-optic material, and mayalso be referred to herein as the optical waveguide modulator 110 orsimply as the modulator 110. The OMC 110 is configured to modulatesignal light 101 received by the OMC 110 at the input optical port 111and to produce modulated light 121, which is transmitted from the mainmodulator output port 117. In some embodiments the modulated light 121may carry useful data and be directed along a data path 128 of anoptical communication system to an optical receiver at another end of anoptical communication link. The bias control port 114 of the OMC 110 isconfigured to receive an electrical bias signal 143 for controlling themodulator set point. The output tap port 118 is configured to providetapped light 123 indicative of the modulator set point. The tapped light123 may be obtained, for example, by tapping off a small portion of themodulated light 121 at an output of the MOC 110. It may also be obtainedby using an optical mixer at the output of the MOC 110 to produce boththe modulated and tapped optical signals 121, 123. In some embodiments,the tapped light 123 may be tapped off at an intermediate location inthe OMC 110.

in one embodiment the optical tap (not shown in FIG. 1) tapping off aportion of the modulated light 121 could be implemented on the same SOIwafer, or other suitable substrate, as the reset of the MOC 110, forexample using a waveguide directional coupler, wherein two waveguidesare brought close enough together such that some optical power mayevanescently couple from the main input waveguide to the tap secondwaveguide. The optical tap could also be implemented using a 2×2MMIdevice with a large splitting ratio, such as for example 95:5, asschematically illustrated in FIGS. 5 and 6.

The OMR 130 has two optical ports, a first optical port 131 opticallyconnected to the output tap port 118 of the OMC 110 for receiving thetapped light 123, and a second optical port 132 for receiving referencelight 103, which may be also be referred to as the local oscillator (LO)light 103 or the amplifying light 103. The OMR 130 may include anoptical mixer (OM) 133 configured to mix the reference light 103 withthe tapped light 123 and to produce one or more mixed light signals eachcombining the reference and tapped light, and a photodetector (PD)circuit (PDC) 140 including one or more photodetectors (PDs) andconfigured to convert the one or more mixed light signals into one ormore electrical feedback signals 141 responsive to changes in themodulator set point.

One advantage of mixing the tapped light 123 with the reference light103 to obtain the electrical feedback signal or signals 141 is theability to amplify the feedback signal when the optical power P_(r) ofthe reference light 103 received by the OMR at the reference input port132 is greater than the tapped optical power P_(s), i.e., the opticalpower of the tapped light 123 received by the OMR 130 at the signalinput port 131. The OMR 130 may be configured so that the electricalfeedback signal or signals 141, denoted herein generally as S, becomesubstantially proportional to the square root of the product P_(r)·P_(s)of the tapped optical power P_(s) and the reference optical power P_(r):S˜√(P_(r)·P_(s)), or to the power product itself: S˜P_(r)·P_(s), orgenerally to a rising function of the product: S˜F{P_(r)·P_(s)}, whereF{x} denotes a function of ‘x’ which value increases when ‘x’ increases.

Thus, by using a higher-power reference light 103, the electricalfeedback signal or signals 141 at the output of the OMR 130 may beamplified relative to a direct detection scheme in the absence of areference signal.

With reference to FIGS. 2A and 2B, the signal light 101 and thereference light 103 may be produced by an optical source which mayinclude one or more light emitters such as for example one or more laserdiodes (LD) or/and one or more other suitable lasers or non-lasing lightemitters, e.g., light emitting diodes (LEDs). In one embodiment, themodulator device 100 may include two optical ports, for example in theform of two planar optical waveguides, for receiving the signal light101 and the reference light 103 separately from two different opticalemitters, for example two different LDs or LEDs 152 a and 152 asillustrated in FIG. 2B. In other embodiments the modulator device 100may include an input optical port, such as for example a planar opticalwaveguide, for receiving input light from a single optical emitter, forexample an LD or a LED, from which both the signal light 101 and thereference light 103 are produced. In one embodiment, the reference light103 may be a delayed portion of the output signal light 121 or of thetapped light 123.

With reference to FIG. 2A, in one embodiment the modulator device 100may include an input optical circuit 150 that is configured to producethe reference light 103 by tapping off a relatively small portion ofinput light 151 using an optical tap 154, with the rest of the inputlight 151 continuing towards the OMC 110 as the signal light 101. Theinput light 151 may be obtained for example from a laser source 152,such as a suitable LD. In some embodiments the input optical circuit 150may further include an optical delay line 156 to approximately equalizethe optical path length of the signal and reference light 101, 103between the laser source 152 and the OMR 150, or to make the differencebetween the respective optical lengths to be less than the coherencelength of the laser source 152. The delay line 156 may be absent inembodiments where the coherence length of the input light 151 issufficiently large, or where the tapped light 123 and the referencelight 103 are mixed in the OMR 130 incoherently. In some embodimentswherein the tapped light 123 and the reference light 103 are coherentlymixed in the OMR 130, an optical phase tuner 158 may further be providedin the optical path of the reference light 103 in order to provide finetuning of the optical phase ϕ_(r) of the reference light 103, asdescribed more in detail hereinbelow. The optical phase tuner 158 may beembodied as known in the art, for example using one or more metalelectrodes disposed over or adjacent to a waveguide formed in anelectro-optic material, as illustrated in FIG. 10. It will beappreciated that the optical phase tuner 158 may also be embodied inother ways, for example using the thermo-optic effect by heating aresistive element within or near-to the core of the waveguide as knownin the art.

Advantageously, in embodiments wherein the insertion loss of the OMC 110is not insignificant, tapping off a small portion of the input light 151prior to the OMC 110 and mixing it with the tapped light 123 providesthe ability to substantially amplify the feedback signal S 141 at thecost of only a small decrease in the useful optical power at the outputof the OMC 110, i.e., the optical power of the modulated signal 121. Byway of example, in an embodiment wherein the electrical feedback signalS is proportional to the product (P_(r)·P_(s)) and the insertion loss ofthe OMC 110 is 10 dB, tapping off 3% of the input light 151 to producethe reference light 103 and 1% of the modulated light 121 to produce thetapped light 123 would result in almost 10 dB gain in the feedbacksignal compared to an equivalent non-mixing tapped light detectionscheme with a 4% tap at the output of the OMC 110, for the same smallincrease in the total insertion loss of the modulator system from theoutput of the optical source 152 to the signal output port 117 of theOMC 110.

With reference to FIG. 3, the optical modulator system of FIG. 1 mayimplement method 300 of automatically controlling a set point or bias ofthe OMC 110. The method 300 includes the following two general steps oroperations: a) at step 320 mixing tapped off modulated light, such asthe tapped light 123, with reference light of a greater power, such asthe reference light 103, to obtain one or more mixed light signalswherein the tapped off modulated light is combined with the referencelight, and b) at step 330 using a PD circuit comprising one or more PDsto convert the one or more mixed light signals into one or moreelectrical feedback signals carrying information about the modulatorbias. In one embodiment the method may include tapping off a portion ofthe modulated light at the modulator output at step 310, and may furtherinclude step 340 of adjusting the modulator bias based on the one ormore electrical feedback signals.

With reference to FIGS. 4A and 4B, the OMR 130 is preferably disposedadjacent to OMC 110, so as to reduce the optical loss therebetween. Boththe OMC 110 and OMR 130 may be enclosed within, or mounted upon, a samehousing 10. In one embodiment, the housing 10 may also enclose the EFC180. In one embodiment the OMR 130 may be formed at least in part in orupon a same semiconductor substrate 20 as the OMC 110. For example, insome embodiment all of the elements of the OMC 110 and OMR 130 may beformed using the Silicon-on-Isolator (SOI) technology in a same SOIchip. In some embodiments, the OMR 130, or a portion thereof, may beformed in a different substrate (not shown) that may be for examplebutt-coupled to the OMC substrate or chip 20. The EFC 180 may beembodied using analog and/or digital electronics, or a combinationthereof. In embodiments wherein one or both of OMC 110 and OMR 130 ofthe modulator device 100 is implemented in a semiconductor chip, the EFC180 may also be implemented fully or in part in the same semiconductorchip, or may be implemented separately therefrom. In some embodiments,the EFC 180 may be embodied as a separate module that may include one ormore dedicated or shared hardware processors or programmable logiccircuits. The EFC 180 may be embodied using analog electrical circuitry,digital electronics, or a combination thereof. Digital electronics thatmay be used to implement the EFC 180 may include one or more FPGAs, oneor more microprocessors, and/or one or more application specificintegrated circuits (ASIC).

Turning now to FIG. 5, there is schematically illustrated a quadraturemodulator (QM) 210 which represents one embodiment of the OMC 110. QM210 may be in the form of an optical waveguide structure that convertsthe input signal light 101 into two modulated optical signals that areconventionally referred to as the I (in-phase) optical signal and Q(quadrature) optical signal, and which are combined together to obtainthe modulated light 121 and the tapped light 123. In the illustratedembodiment the QM 210 is embodied as a Mach-Zehnder (MZ) interferometer(MZI) having an optical modulator 112 in each of its two arms thatproduce the I and Q optical signals, and an output optical coupler 116for combining the I and Q optical signals produced by the opticalmodulators 112. The QM 210 further includes a first tunable opticalphase shifter 116 that is controlled through the electrical control port114 to adjust the optical phase of one of the I and Q signals so thatthey are added in the output coupler with an IQ phase shift ϕ_(IQ)therebetween, which defines a set point of the QM 210. The outputoptical coupler 116 may be for example in the form of a 2×2 multi-modeinterference (MMI) coupler that mixes the I and Q optical signalsreceived at it input ports and outputs the modulated light 121 and thetapped light 123 from its output ports 117 and 118, respectively, eachof them combining the I and Q optical signals with the IQ phase shiftϕ_(IQ) therebetween. The output optical coupler 116 may also havealternative embodiments, for example it may be in the form of a 2×1optical combiner followed by a 1×2 optical splitter or tap.

In order to ensure proper operation of the QM 210, the IQ phase shiftϕ_(IQ) imposed by the first tunable optical phase shifter 116 should beset to a desired set-point value ϕ_(IQ) ⁰. In example embodimentsdescribed hereinbelow, the desired set-point value ϕ_(IQ) ⁰ of the IQphase shift ϕ_(IQ) is equal substantially to π/2 rad, so as to ensurethat the I and Q optical signals in the QM 210 are added in quadratureat the output of the QM 210; however, the particular desired value ofthe optical phase shift ϕ_(IQ) ⁰ may differ in other embodiments, andall such values are within the scope of the present disclosure. Thevalue of the IQ phase shift ϕ_(IQ) is controlled by an electric biassignal 199, which is provided at the control port 114 and which may beadjusted in operation in response to a drift in modulator properties soas to maintain the modulator at a desired set point ϕ_(IQ)=ϕ_(IQ) ⁰.

In one embodiment the QM 210 may be configured as a QPSK modulator, withthe optical modulators 112 in cooperation with the tunable opticalshifter 116 producing two BPSK modulated I and Q optical signals,resulting in an equidistant QPSK symbol constellation at the QM outputsof when added with the IQ phase shift ϕ_(IQ) ⁰=π/2.

Referring to FIG. 6, in one embodiment each of the optical modulators112 may be in the form of a Mach-Zehnder modulator (MZM) 212, so as toform a nested MZM wherein two parallel MZMs 212 are nested in an MZI232. In operation, each of the MZM 212 may be driven by an RF signal inthe form of a nonreturn-to-zero (NRZ) binary voltage waveform V_(I,Q)(t)that switches between +Vπ and −Vπ, thereby producing the I and Q opticalsignals that are BPSK modulated. Here, Vπ is the half-wave voltage, i.e.the voltage that causes the optical phase of the light propagatingthough the MZM to change by π radian relative to zero voltage. Theoutput lights 121, 123 are both QPSK modulated when the two MZMs 112 arebiased at their null transmission points, corresponding to MZM biasvoltages V_(MZM1,2)=±Vπ, ±3Vπ, ±5Vπ, . . . , applied to their respectivebias electrodes, and the MZI 232 is biased at the quadrature phase, i.e.ϕ_(IQ)=π/2, which corresponds to an IQ bias voltage V_(IQ) of(Vπ)/2±nVπ, n=0, 1, . . . applied to the bias electrode 148 implementingthe tunable IQ phase shifter 116. It will be appreciated that theoptical modulators 112 capable of optical BPSK modulation may beembodied using modulator structures other than an MZM, including but notlimited to ring waveguide modulators.

It will be appreciated that the method 300 illustrated in FIG. 3 whereina feedback signal for automatic bias control is amplified by mixing amodulator tapped-off signal with a more powerful reference signal may beemployed in various modulator circuits and to control bias voltages atdifferent locations in the modulator circuit and to control set-pointsof various constituent modulators. For example, it may be used tocontrol bias voltages of the two MZMs 212 in the OMC embodiment of FIG.4, for example using one of known MZM control algorithms based onmonitoring an average output power from the QM, or by monitoring opticalpower tapped off at the output of the MZMs prior to the output couplerof the outer MZI.

Turning now to FIG. 7, there is illustrated an embodiment of the OMR 130that is denoted as OMR 130 a. In this embodiment the OM 133 is in theform of an optical hybrid (OH) 233 with two input ports and four outputports, while the PDC 140 includes four PDs 135 that are opticallycoupled to the four output ports of the OH 133 and that are followed bytwo summing circuits 136, each of which configured to sum electrical PDsignals from two of the PDs to produce two electrical feedback signals141 a and 141 b. Input optical ports 131, 132 of the OH 233 areoptically coupled to the OMC 110 to receiving the tapped light 123 andthe reference light 103, which are combined in the OH 233 so as tooutput four mixed light signals 134 _(n), n=1, 2, 3, or 4, combining thetapped and reference light 123, 103 with a progressively increasingoptical phase shift ϕ_(OH) therebetween. In one embodiment the OH 233 isa 90° optical hybrid and outputs the optical mixed signals 134 _(n),n=1, 2, 3, or 4, wherein the tapped and reference light 123, 103 arecombined with an incremental optical phase shift therebetweenϕ_(OH)=π/2·(n−1). The summing circuits 136 may be in the form ofdifferential amplifiers that output differential PD signals that areproportional to a difference between their inputs, i.e.,J_(diff1)˜(J₁−J₂), and J_(diff2)˜(J₃−J₄), or generally to a monotonicfunction of the difference. Here J_(n), n=1, 2, 3, or 4, denote theelectrical PD signals at the output of the PDs 135, for examplephoto-currents or photo-generated voltages. Assuming that the phaseerror of the OH 233 is small, the outputs of the PD summing circuits 136may be written in the following form:

J _(diff,1) =R√{square root over (P _(ref) P _(Sig)(t))}cos(Δϕ(t))  (1)

J _(diff,2) =R√{square root over (P _(ref) P _(Sig)(t))}sin(Aϕ(t))  (2)

Here P_(ref) is the optical power of the reference light 103,P_(sig)=P_(sig)(t) is the optical power of the tapped light 123, andΔϕ(t) is the optical phase difference between the reference and tappedlight at the point of combining, R is a proportionality coefficient thataccounts for the PD conversion efficiency and possible gain in thesumming circuits 136, and t denotes time. The differential PD signalsJ_(diff1) and J_(diff2) at the output of the differential summingcircuits 136 will also be referred to herein as the I and Q electricalsignals, respectively, and denoted as J_(I) and J_(Q).

Turning now to FIG. 8, there is illustrated an exemplary implementationof the OMR 130 a of FIG. 5 wherein the PDs 135 are implements as fourphotodiodes 235 that are electrically connected to implement twobalanced photodetectors in the form of balanced PD pairs 236, eachbiased by a voltage source, with the electrical currents J_(I) and J_(Q)supplied by the voltage sources being equal to the difference betweenthe two photocurrents generated by the PDs in the respective balanced PDpair. The balanced PD pairs 236 provide the differential summingfunctionality that is represented by the summing circuits 136 in FIG. 7.

Turning now to FIG. 9, one embodiment of the OMR 130, which is denotedas 130 b, includes two rectifying circuits 138 which connect to theoutputs of two balanced PD pairs 236 and which are configured to rectifythe differential PD signals J_(diff1) and J_(diff2) received from theoutput of the balanced PD pairs 236 prior to outputting them as theelectrical feedback signals 141 a and 141 b. The balanced PD pairs 236may be embodied for example using differential amplifiers, or asillustrated in FIG. 8, or in any other suitable way. In one embodiment,the rectifying circuits 138 are broad-band and fast enough to respond tochanges in the received signal at the data rate of the modulation. Inone embodiment, the rectifying circuits 138 may be configured to outputsignals that are proportional to squares of the differential PD signals(J_(diff1))²˜(J₁−J₂)² and (J_(diff2))²˜(J₃−J₄)². In one embodiment, therectifying circuits 138 may be configured as high-speed RF powerdetectors that are capable of responding to changes in the input RFpower at the modulation data rate.

Referring now to FIG. 10, there is illustrated an exemplary opticalmodulation system with an automatic modulator bias control. It includesan embodiment of the modulator device 100 in the form of a modulatordevice 400, and an EFC 280 embodying the EFC 180 of FIG. 1. Themodulator device 400 may be embodied as a PIC that is formed fully or inpart in or upon a semiconductor substrate 422, and may be referred to asthe PIC modulator device 400 or simply as the PIC 400. The PIC modulatordevice 400 may be generally as described hereinabove with reference toFIGS. 1, 2A, 5, and 6; with the input optical splitter or tap 154, theoptical phase tuner 158 in the path of the reference light 103, and QM210 embodying OMC 110. The QMR 230 is an embodiment of the OMR 130 ofFIG. 1 and may be, for example, in the form of the QMR 130 b illustratedin FIG. 9, but other OMR embodiments may also be possible, with someexamples described hereinbelow. The input optical tap 154, whichconnects at its input port to the input optical waveguide 401 of the PIC400, has a main output port that connects to the input optical waveguide402 of the QM 210. An optical waveguide 403 connects a tap port of theinput optical tap 154 to a second optical port of the OMR 230 andincorporates the optical phase tuner 158. The QM 210 has a main outputport that connects to an output optical waveguide 404 of the PIC 400,and a tap port that connects to a first optical port of the OMR 230.

In operation input light 151 received into the input optical waveguide401 is split by the tap 154 into the reference light 103 that isdirected towards the second optical port of the OMR 230, and the signallight 101 that is coupled into the QM 210 to be modulated. The QM 210outputs modulated light 121 and tapped light 123, with the formerprovided through the output waveguide 404 as the main output of the PICmodulator device 400, and the tapped light 123 guided into the firstinput optical port of the OMR 230. OMR 230 is configured to mix light103 tapped off before the QM 210 and light 123 tapped off at the outputof the QM 210 and to produce, from the mixed light, one or moreelectrical feedback signals 141, such as for example two quadratureelectrical feedback signals 141 a and 141 b as described hereinabovewith reference to FIGS. 7-9.

The EFC 280 may be configured to process the feedback signal or signals141 and to generate therefrom the bias control signal 199 forcontrolling the bias voltage Vb that determines the IQ phase shiftϕ_(IQ) in the QM 210, and a reference control signal 198 for tuning theoptical phase of the reference light 103 by means of the optical phasetuner 158. Accordingly, in one embodiment the EFC 280 may include areference control circuit (RCC) 286 and a bias control circuit (BCC) 285that are configured to process the feedback signal or signals 141 and togenerate the reference control signal 198 and the bias control signal199, respectively. The EFC 280 may implement a variety of controlalgorithms to track changes in the modulator set point and to ensurethat the IQ phase shift stays approximately equal to the desiredset-point value, such as π/2 rad in a typical embodiment. In oneembodiment, the EFC 280 may be configured to vary the optical phase ofthe reference light 103 by varying the reference control signal 198 tothe input optical phase tuner 154 while maximizing or minimizing a firstelectrical feedback signal 141 a, and to tune the bias control signal199 to vary the voltage Vb that controls the IQ phase shift so as toequalize two electrical feedback signals 141 a and 141 b. In oneembodiment, the EFC 280 may be configured to vary at least one of theoptical phase of the reference light 103 and the IQ phase shift in theQM 210 so as to equalize the electrical feedback signals 141 a and 141b. Other embodiments of the control algorithm will become clear from thedescription hereinbelow.

Principles of operation of the EFC 280 may be understood by consideringan embodiment wherein the QM 210 is an optical QPSK modulator whereinoptical fields E_(I)(t) and E_(Q)(t) are added at the output with thephase shift ϕ_(IQ) to produce the tapped light 123, which is thencoherently mixed with the reference light 103 in the OH 233, asillustrated in FIGS. 7-9 to produce four mixed optical signals 134 nwith complex amplitudes

E ₁=(E _(sig) +E _(ref))  (3a)

E ₂=(E _(sig) −E _(ref))  (3b)

E ₃=(iE _(sig) +E _(ref))  (3c)

E ₄=(iE _(sig) −E _(ref))  (3d)

where i=√−1, E_(Sig)=|E_(Sig)|exp(iϕ_(Sig)) is the complex amplitude ofthe tapped light 123 in the OH 233, E_(ref)=|E_(ref)|exp(iϕ_(ref)) isthe complex amplitude of the reference light 123 in the OH 233. Assumingthat the optical fields E_(I)(t) and E_(Q)(t) each have a phase thatswitches between 0 and π and a same real value amplitude, i.e.

|E _(I)(t)|=|E _(Q)(t)|=A,  (4)

The complex amplitude E_(Sig) of the tapped light 123 may be describedby a four-point constellation defined by the following two equations (2)and (3), see FIG. 11:

$\begin{matrix}{{\varphi_{Sig} = {\left\lbrack {0,\frac{\pi}{2},\frac{3\; \pi}{2},\pi} \right\rbrack + \frac{~\varphi_{IQ}}{2}}},} & (5) \\{and} & \; \\{{E_{Sig}} = {{A\left\lbrack {{\cos \left( \frac{\varphi_{IQ}}{2} \right)},{\sin \left( \frac{\varphi_{IQ}}{2} \right)},{\sin \left( \frac{\varphi_{IQ}}{2} \right)},{\cos \left( \frac{\varphi_{IQ}}{2} \right)}} \right\rbrack}.}} & (6)\end{matrix}$

The four values within brackets [ . . . ] in the RHS of equations (5)and (6) denote four possible values of the real-valued amplitude|E_(Sig)| (eq.6) and phase ϕ_(Sig) (eq.5) of the optical fieldE_(Sig)(t) of the tapped light 123 that result from the BPSK modulationof the I and Q optical signals in the QM 210; JE_(re)f is thereal-valued amplitude of the reference light 103 and ϕ_(ref) is theoptical phase thereof in the OH 133 relative to that of the tapped light123, |x| denotes absolute value of ‘x’. The constellation described byequations (4) and (5) is illustrated in FIG. 11.

The OH 233 combines the tapped light 123 with the reference light 103,and outputs the four different mixed optical signals wherein the tappedlight is coherently mixed with the reference light with a phase shiftn·π/2, with complex amplitudes defined by equations (3a)-(3d).

The in-phase (I) and quadrature (Q) electrical signals J_(I) and J_(Q)at the output of the differential summers 136 are given by equations (1)and (2) with P_(Sig)=|E_(Sig)(t)|², P_(ref)=|E_(ref)(t)|², and

Δϕ(t)=ϕ_(Sig)(t)+ϕ_(ref),  (7)

with ϕ_(Sig)(t) and |E_(Sig)(t)| switching between four values given byequations (5) and (6). Generally, these signals depend on the IQ phaseϕ_(IQ), and therefore are sensitive to its variations from the desiredset-point value ϕ_(IQ)=π/2. However, it can be seen that these signalscease to depend on the IQ phase ϕ_(IQ) after averaging over a timeT_(avrg) that is much greater than the duration T_(sym) of one BPSKsymbol, which is defined by the inverse of the modulation data rateR_(mod). By way of example, R_(mod) may be in the gigabit per second(Gb/s) range, for example 10-100 Gb/s.

Accordingly, the differential PD signals J_(I) and J_(Q) may be firstrectified by the rectifying RF circuits 138 that operate at themodulation data rate R_(mod) if lower-speed electronics is to be used inEFC 280 to detect and track changes in the IQ phase ϕ_(IQ). The OMR 230therefore may include the rectifying RF circuits 138, for example in theform of high-speed squaring circuits, such as RF power detectors.Indeed, time-averaged power P_(I)=<(J_(I))²> and P_(Q)=<(J_(Q))²> of thedifferential PD signals J_(I) and J_(Q) may be described by thefollowing equations (8) and (9):

P _(I) =aP _(Sig) P _(ref)·[1+cos(ϕ_(IQ))·cos(ϕ_(IQ)−2ϕ_(ref))]  (8)

P _(Q) =aP _(Sig) P _(ref)·[1−cos(ϕ_(IQ))·cos(ϕ_(IQ)−2ϕ_(ref))];  (9)

they are sensitive to ϕ_(IQ) and therefore can be used as the feedbacksignals 141 a and 141 b by a lower-speed electronics in the EFC 280.Here a is a phase-independent multiplier coefficient that depends on thePD conversion efficiency and gain and/or efficiency parameters of theelectrical circuitry following the PDs in the OMR 230. Equations (8) and(9) are obtained assuming that all QPSK symbols appear in the tappedsignal 123 with equal frequency during the time of averaging.

From equations (8) and (9) it may be observed that the time-averagedsignals P_(I) and P_(Q) are equal at the desired quadrature set pointfor the IQ phase shift in the QM 230, i.e. when

ϕ_(IQ)=π/2+π·m,  (10)

and also when

ϕ_(IQ)=2ϕ_(ref)+π/2+π·m,  (11)

where m is an integer. The same may also hold for alternativeembodiments of the rectifying circuits 138, for example when theiroutput signals are proportional to absolute values of their inputsrather than squares thereof. Accordingly, in one embodiment the EFC 280may be configured to compare outputs of the rectifying circuits 138 atfrequencies significantly lower than the modulation data rate, e.g. thetime-averaged RF powers P_(I) and P_(Q) of the differential PD signalsJ_(I) and J_(Q), and adjust the bias control signal 198 so as to keep afeedback signal difference Δ=|P_(I)−P_(Q)| between them below a suitablysmall value.

Referring to FIG. 12, the modulator system illustrated in FIG. 10 mayimplement a modulator bias control method 500 that includes thefollowing steps or operations: a) taping off a fraction of modulatorinput light prior to the QM 210 to obtain reference light at step 510;b) combining light from the output of the QM with the reference lightusing a 90° OH at step 530 to obtain four mixed optical signals; c)converting the four mixed optical signals into two quadrature electricalsignals J_(I) and J_(Q) at step 530; d) rectifying the quadratureelectrical signals J_(I) and J_(Q) at step 540; and, e) adjusting the IQphase shift ϕ_(IQ) so as to equalize low-frequency components of therectified quadrature electrical signals J_(I) and J_(Q) at step 550.

In one embodiment, the EFC 280 may further be configured to monitor thesignal difference Δ while varying the reference control signal 198 tochange the relative optical phase ϕ_(ref) of the reference light 103, soas to ensure that the signal difference does not depend on the referencecontrol signal and hence is independent on ϕ_(ref). A signal differenceΔ=|P_(I)−P_(Q)| that stays substantially at zero while the referencecontrol signal applied to the input optical phase shifter varies in asufficiently wide range indicates that the IQ bias voltage V_(IQ) in theQM 210 is equal substantially to Vπ/2, i.e. corresponds to the desiredquadrature set point ϕ_(IQ)=π/2+π·m of the QM 210, as defined byequation (10).

Referring now to FIG. 13, there is illustrated a functional blockdiagram of an EFC 380 that may embody OFC 280 of FIG. 10. OFC 380 isconfigured to implement step 550 of the method 500. It includes acomparator module 283 that connects to a decision module 284, which isin turn operationally coupled to a bias control module 285, and mayfurther be operationally coupled to a reference control module 286. Thecomparator module 283 has two input ports for connecting to output portsof the OMR 230 for receiving the electrical feedback signals 141 a and141 b in the form of the rectified quadrature electrical signals J_(I)and J_(Q). The comparator 283 may be preceded by averaging circuits 272,which may be for example in the form of low-pass (LP) filters 272 thatlet through only low-frequency components of the feedback signals 141a,b below a filter cut-off frequency f_(LF)<<R_(mod) of the OMC 210. Byway of example, f_(LF) may lie for example in the MHz or, preferably,kHz range. In some embodiments the averaging or LP filters 272 may beprovided at the output of the OMR 230, or by low-frequency connectingcircuitry between the OMR 230 to the EFC 280. The functionality of theaveraging filters 272 may also be effectively provided by low-frequencycircuitry of the comparator 283.

In operation, the comparator 283 compares the averaged rectified firstand second feedback signals 141 a and 141 b so as to evaluate a signaldifference in the I and Q channels of the OMC 230, and communicatesresults to the decision module 284, which may signal to the bias controlmodule 285 to adjust the IQ bias Vb in the QM 230 if the inputs to thecomparator 283 is found to differ by more than a pre-defined errorthreshold e₀. For example, the comparator 283 may output an error signale that is proportional to the difference Δ between the average RF powersP_(I) and P_(Q) of the differential PD signals J_(I) and J_(Q),e˜Δ=(P_(I)−P_(Q)), and the decision module 284 may send a signal to thebias control module 285 to change the IQ bias voltage Vb for adjustingϕ_(IQ) if |e|>e₀. If |e|<e₀, the decision module 284 may keep the biasvoltage Vb unchanged. In one embodiment, the reference control module198 may be operable to vary the reference optical phase ϕ_(ref) in apre-defined range, such as by suitably varying the phase referencecontrol signal 198, so as to ensure that the error signal e from thecomparator 283 remains below the error threshold e₀ for any referencephase value ϕ_(ref). By way of example the reference control module maybe configured to vary the reference control signal 198 so that the errorsignal e is determined for a plurality of values of the reference phaseϕ_(ref) that spans about 90°, or a fraction thereof. In one embodiment,the reference control module 286 may dither the reference phase ϕ_(ref)using a suitable dither signal, and the decision module 284 may beconfigured to detect the dither signal, or a signature thereof, in theerror signal e, and to vary the IQ bias voltage Vb so as to minimize thepresence of the dither signal or its signature in the error signal atthe output of the comparator 283.

Furthermore from equations (8) and (9) follows that the time-averagedsignals P_(I) and P_(Q) both cease to dependent on the reference phaseϕ_(ref) when equation (10) is satisfied, i.e. at the desired quadratureset point for the IQ phase shift in the QM 210. Accordingly, in oneembodiment either one of the time-averaged I and Q electrical signalsP_(T) and P_(Q) may be monitored while varying the relative opticalphase of the reference light ϕ_(ref), and changing the bias controlsignal 199 to adjust the IQ phase shift ϕ_(IQ) if the monitored signalP_(I) or P_(Q) changes in dependence on the reference control signal 198that controls the optical phase ϕ_(ref) of the reference light 103.

Accordingly, embodiments wherein the rectifying circuits 138 of the OMR230 are squaring circuits, for example are configured as RF powerdetectors, the EFC 280 may be configured to vary the optical phase ofthe reference light ϕ_(ref) while monitoring a time average of one ofthe first and second electrical feedback signals from the outputs of therectifying circuits 138, i.e. one of the average RF powers P_(T) andP_(Q) of the differential PD signals J_(I) and J_(Q). The EFC 380 maythen further be configured to adjust the bias control signal 199 so asto keep either one of the average RF powers P_(I) or P_(Q) substantiallyindependent on the optical phase of the reference light ϕ_(ref). Thismay include for example using the first tunable optical phase shifter116 to adjust the IQ phase shift ϕ_(IQ) in the QM 210 if the firstelectrical feedback changes in dependence on the optical phase of thereference light.

Referring to FIGS. 14A and 14B, there illustrated functional blockdiagrams of two exemplary embodiment of the EFC 280, EFC 280 a and EFC280 b, that are configured to control the IQ phase shift of the QM 230so as to make a time-averaged or LP-filtered electrical feedback signal241 insensitive to variations in the reference phase ϕ_(ref). Theelectrical feedback signal 241 may be, for example, the RF power P_(I)141 a or P_(Q) 141 b at the output of either of the I and Q channels ofthe PDC 240 in FIG. 9. It may also be an electrical feedback signal 141at the output of an OMR 230 a that is based on a 180° optical mixer asillustrated in FIG. 15.

Turning first to FIG. 14A, in one embodiment the EFC 280 a may beconfigured to operate similarly to the EFC 380 of FIG. 13, but with theinput circuit thereof configured to detect changes in the LP-filteredelectrical feedback signal 241. The comparator 282 compares the outputsignal from the LP filter 272 with a delayed version thereof, andoutputs a signal that is indicative of their difference to detectlow-frequency changes in the electrical feedback signal 241. Thedecision module is configured to adjust the IQ bias voltage Vb of the QM230 so as to minimize the signal it receives from the comparator 282. Inoperation, the reference control module 276 generates the referencecontrol signal 198 so as to vary the reference optical phase ϕ_(ref) ina desired range, for example over about π/2 rad, while the decisionmodule 284 monitors for changes in the received signal 241, e.g., P_(I)or P_(Q), that may be caused by the changes in the reference phase, andsignals to the bias control module 275 to adjust the IQ bias of the QM210 or QM 110 so as to minimize the output signal from the comparator282, so as to search for the IQ bias voltage Vb that makes theelectrical feedback signal 241 after the LP filter 272 insensitive tothe reference phase ϕ_(ref).

Turning now to FIG. 14B, in one embodiment the reference control signal198 may be dithered by the reference control module 276, i.e., modulatedwith a desired dither waveform that is chosen to dither the referenceoptical phase ϕ_(ref) in the desired range, and the monitored signal241, e.g. P_(I) or P_(Q), analyzed for the presence of the ditherwaveform using a dither detector 271 while varying the bias controlsignal 199, so as to find the bias control signal 199 corresponding tothe absence of the dither waveform in the monitored signal 241. Forexample, the reference control module 276 may be configured to modulatethe reference phase F_(ref) at a suitably low dither frequency f_(d),for example 1-10 kHz, and the dither detector may be embodied as anarrow-band filter centered at the dither frequency f_(d). The decisionmodule 284 may be configured to adjust the IQ bias voltage Vb of the QM230 so as to minimize the signal it receives from the dither detector271. The narrow-band filter in the dither detector 271 may be embodiedfor example by a digital or analog lock-in detector, as is known in theart.

In one embodiment, the bias control signal 199, for example the biasvoltage Vb, may be modulated about a dc bias value <Vb> at a suitablylow dither frequency f_(d). The EFC 280 may then be configured to vary adc component <Vb> of the bias voltage in a predefined range so as tomaximize a second harmonic of the dither frequency, i.e. 2f_(d), in themonitored signal P_(I) or P_(Q). The second harmonic of the ditherfrequency, i.e. 2f_(d), in the monitored signal may be measured byfiltering with a narrow-band filter 271 centered at 2f_(d).

Referring now to FIG. 15, it will be appreciated that in someembodiments the OMR 230 may be replaced with the OMR 230 a in which onlyone channel of the two marked in FIG. 9 as “I channel” and “Q channel”is retained. In such embodiments, the 90° OH 233 may be replaced with a180° degree 2×2 optical mixer 333 that outputs two optical mixed signals134 ₁ and 134 ₂ in which the reference light 103 is added to the tappedlight 123 with an optical shift that differs between the two opticalmixed signals 134 _(1,2) by 180°, so that if the input electrical fieldsE₁ and E₂ are added in the first output mixed signal 134 ₁, they aresubtracted in the second output mixed signal 134 ₂. The first and secondmixed optical signals 134 _(1,2) are then differentially detected usinga balanced PD 236, generally as described hereinabove with reference toFIGS. 7-9, and the resulting differential PD signal 137 is passed to theRF squaring rectifier 138, such as for example an RF power meter that isresponsive to changes in the input signal at the modulation rate R_(mod)as described hereinabove. An output of the RF squaring rectifier 138forms the electrical feedback signal 141 that is then passed to the EFC280, for example as embodied in FIGS. 13, 14A and 14B.

Referring now to FIG. 16, in one embodiment the OMR 130 of FIG. 1 may beembodied as OMR 230 b that is generally as described hereinabove withreference to FIG. 9, but in which the outputs of the squaring rectifiers138 summed, which may result in an electrical feedback signal 141 P(t)being substantially independent on either the IQ phase shift ϕ_(IQ) orthe reference phase shift ϕ_(ref), but still proportional to the opticalpower of the reference light P_(ref), as could be seen for example fromequations (1) and (2):

P(t)=P _(I) +P _(Q)=2RP _(Sig) P _(ref)  (9)

Accordingly, the OMR 230 b of FIG. 16 may be used to provide additionalgain, ˜P_(ref), to the feedback signal for controlling modulator bias inembodiments wherein the tapped and reference light 123, 103 are mutuallyincoherent, for example produced by two different optical emitters, orfrom a same optical emitter but with a relative optical delay thatexceeds the coherence length of the emitter; it will be appreciated thatthe optical phase tuner 156 may be omitted in such embodiments. The OMRof FIG. 16 may be used to control for a modulator bias drift thatresults in changes of the average output optical power from themodulator, for example to control the bias of an MZM.

The OMRs 130, 230, 230 a, and 230 b may be embodied in fully orpartially in the same PIC chip as the respective OMC 130 or 230, or theymay be embodied in a different chip. The EFCs 180 and 280 may beembodied using analogue electrical circuits or they may be embodiedusing suitably programmed digital processors, or using programmablehardware logic as known in the art. In embodiments using analogueelectronics, the comparator 283 may be embodied using a differentialamplifier, and the decision module 284 may be embodied using for examplea PID control circuit as known in the art. Alternatively,functionalities represented by elements of the EFC shown in FIGS. 10,13-16 may be embodied using a digital processor.

Referring now to FIG. 17, in one embodiment the modulator system of FIG.10 may be embodied using a digital processor 470, a PIC 450 and an RFcircuit 460. The PIC 450 may include at least the QM 210, and may alsoinclude the input tap 154, the reference phase tuner 158, and an opticalmixer 133, 233, or 333. The RF circuit 460 may include one or morecomponents of the PDC 140 or 240, and in some embodiments the RFrectifiers 138. In one embodiment, both the PIC 450 and the RF circuit460 may be embodied in a same semiconductor substrate 422, for exampleas a single SOI chip.

The RF rectifier or rectifiers 138 may be embodied, for example, as oneor more silicon or germanium pn-junction diodes, resistive, andcapacitive elements. Referring to FIGS. 18A and 18B, example rectifiercircuits may pass the output current from the photodiodes into atransimpedance amplifier followed by a DC-blocking capacitor. The outputof the DC-blocking capacitor may then be connected to either a half-waverectifier using a single diode (FIG. 18A), a full-wave bridge rectifiercircuit (FIG. 18B), or other similar rectifying circuit whose output isproportional to the AC input amplitude. The rectifying circuit then mayoptionally be followed by an RC filter, or other similar filter, toimprove the rectification signal integrity. The transimpedanceamplification could be achieved, for example, by a resistor placed inseries with the photodiode, as illustrated by the resistor R_(T) inFIGS. 18A and 18B. All of these components are possible to realize on anSOI wafer.

Turning back to FIG. 17, the one or more electrical feedback signalsfrom the output of the RF circuit 460 may be digitized by an ADC 465 andpassed to the processor 470 which is configured, for example programmed,to perform the bias control algorithm which example embodiments aredescribed hereinabove with reference to FIGS. 10-16.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentinvention. Indeed, various other embodiments and modifications to thepresent disclosure, in addition to those described herein, will beapparent to those of ordinary skill in the art from the foregoingdescription and accompanying drawings. Thus, such other embodiments andmodifications are intended to fall within the scope of the presentdisclosure. For example, it will be appreciated that semiconductormaterials other than silicon, including but not limited to compoundsemiconductor materials of groups commonly referred to as A3B5 and A2B4,such as GaAs, InP, and their alloys and compounds, may be used tofabricate the PIC modulator device example embodiments of which aredescribed hereinabove. In another example, although example embodimentsdescribed hereinabove may have been described primarily with referenceto an optical waveguide QPSK modulator, it will be appreciated thatprinciples and device configurations described hereinabove withreference to specific examples may be adopted to perform an automaticbias control of optical waveguide modulators of other types, includingbut not limited to multilevel optical QAM modulators. Furthermore, PICmodulator devices example embodiments of which have been describedhereinabove, in other embodiments it may include other optical devices,such as for example, but not exclusively, optical amplifiers.

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

1-20. (canceled)
 21. A modulator device comprising: an optical modulatorcircuit (OMC) configured to modulate signal light to produce modulatedoutput light, the OMC including: an input port for receiving the signallight, an output port for transmitting the modulated output light, and abias control configured to receive an electrical bias signal forcontrolling a set point of the OMC, and an optoelectronic feedback loopcomprising: an optical tap port configured to provide tapped light fromthe OMC indicative of the set point of the OMC; an optical mixer (OM)comprising: a first optical port optically coupled to the optical tapport for receiving the tapped light, and a second optical port forreceiving reference light, the optical mixer configured to mix thereference light with the tapped light to produce at least one mixedlight signal, each of the at least one mixed light signal combining thereference light and the tapped light; a photodetector (PD) circuitcomprising one or more photodetectors (PDs) and configured to convertthe at least one mixed light signals into at least one electricalfeedback signal responsive to changes in modulator set point; and anelectrical feedback circuit (EFC) connected between the PD circuit tothe bias control, and configured to generate the electrical bias signalin dependence on the at least one electrical feedback signal.
 22. Themodulator device according to claim 21, further comprising: an opticalsplitter disposed to receive input light and configured to split theinput light into the signal light and the reference light, and a firstoptical phase tuner configured to tune an optical phase of the referencelight in response to a reference control signal; wherein the electricalfeedback circuit is configured to vary the electrical bias signal andthe reference control signal in dependence on the at least oneelectrical feedback signal.
 23. The modulator device according to claim21, wherein the OMC includes a quadrature modulator circuit connectedbetween the input port and the output port and configured to produce anin-phase modulated optical signal and a quadrature modulated opticalsignal from the signal light, and to combine the in-phase modulatedoptical signal with the quadrature modulated optical signal to obtainthe modulated output light and the tapped light, each of the modulatedoutput light and the tapped light comprising the in-phase modulatedoptical signal and the quadrature modulated optical signal with an IQphase shift therebetween; and wherein the bias control includes a secondoptical phase tuner configured to vary the IQ phase shift responsive tothe at least one electrical bias signal, wherein the IQ phase shiftdefines the set point.
 24. The modulator device according to claim 23,wherein the optical mixer is configured to output the at least one mixedlight signals comprising: a first mixed optical signal combining thetapped light and the reference light with a first optical phase shifttherebetween, and a second mixed optical signal combining the tappedlight and the reference light with a second optical phase shifttherebetween.
 25. The modulator device according to claim 24, whereinthe PD circuit comprises: a first balanced PD receiver configured toproduce a first differential PD signal from the first mixed light signaland the second mixed light signal; and a first radio frequency (RF)rectifier disposed to receive the first differential PD signal andconfigured to produce the at least one electrical feedback signalcomprising a first feedback signal, wherein the first RF rectifier isresponsive to data-rate changes in the first differential PD signal. 26.The modulator device according to claim 25, wherein the first RFrectifier comprises an RF squaring detector configured to produce thefirst feedback signal that is proportional to a square of the firstdifferential PD signal, and wherein the EFC comprises a processorcomprising logic configured to monitor changes in the first feedbacksignal, and to adjust the electrical bias signal so as to reduce thechanges in the first feedback signal.
 27. The modulator device accordingto claim 25, wherein: the optical mixer comprises a 90° optical hybridconfigured to output four mixed optical signals comprising the firstoptical signal, the second optical signals, and further comprising thirdand fourth optical mixed signals, the PD circuit further comprises asecond balanced PD receiver disposed to receive the third and fourthmixed optical signals and configured to produce a second differential PDsignal therefrom, the PD circuit further comprising a second RFrectifier disposed to receive the second differential PD signal andconfigured to produce a second feedback signal, wherein the second RFrectifier is responsive to data-rate changes in the second differentialPD signal.
 28. The modulator device according to claim 27, wherein theEFC comprises a processor comprising logic configured to adjust theelectrical bias signal so as to equalize the first feedback signal andthe second feedback signal.
 29. The modulator device according to claim28, wherein the logic is configured to obtain a difference signal thatis indicative of a difference between the first feedback signal and thesecond feedback signal, and to monitor changes in the difference signal.30. The modulator device according to claim 21, wherein the referencelight includes an optical power greater than the tapped light foramplifying the at least one feedback signal.
 31. A method of modulatesignal light to produce modulated output light using an opticalmodulator circuit (OMC) comprising an input port for receiving thesignal light, an output port for transmitting modulated output light, abias control configured for receiving an electrical bias signalcontrolling a set point of the OMC, the method comprising: a) tappinglight from the OMC to obtain tapped light indicative of the set point ofthe OMC; b) mixing the tapped light with reference light of a greaterpower in an optical mixer to produce one or more mixed light signals,each combining the tapped light with the reference light; c) convertingthe one or more mixed light signals into one or more electrical feedbacksignals comprising information about the set point of the OMC, using aphotodetector (PD) circuit; and d) generating the electric bias signalin dependence on the one or more electric feedback signals using anelectric feedback circuit (EFC).
 32. The method according to claim 31,further comprising: splitting input light into the signal light and thereference light using an optical splitter prior to modulating the signallight by the OMC; tuning an optical phase of the reference light usingan optical phase tuner in response to a reference control signal; andvarying the reference control signal in dependence on the one or moreelectrical feedback signals.
 33. The method according to claim 31,wherein the OMC comprises a quadrature modulator configured to modulateand combine an in-phase modulated optical signal and a quadraturemodulated optical signal; wherein the bias control comprises a firstoptical phase shifter, configured to vary an IQ phase shift between thein-phase modulated optical signal and the quadrature modulated opticalsignal, wherein the IQ phase shift defines the set point of the OMC. 34.The method according to claim 33, wherein the step a) comprisesobtaining a first mixed light signal and a second mixed light signal,wherein the tapped light is added to the reference light with a phaseshift that differs generally by 180° between the first mixed lightsignal and the second mixed light signal.
 35. The method according toclaim 34, wherein the step b) comprises differentially detecting thefirst mixed light signal and the second mixed light signal to obtain afirst differential PD signal, and rectifying the first differential PDsignal using an RF rectifier that is responsive to data-rate changes inthe first differential PD signal to obtain a first electrical feedbacksignal of the at least one electrical feedback signals.
 36. The methodaccording to claim 35, wherein the step c) comprises detecting anaverage of the first electrical feedback signal over time.
 37. Themethod according to claim 35, wherein the first RF rectifier comprisesan RF squaring detector configured to produce the first feedback signalthat is proportional to a square of the first differential PD signal,and wherein the EFC is configured to monitor changes in the firstfeedback signal that are responsive to the reference control signal, andto adjust the electrical bias signal so as to reduce the changes. 38.The method according to claim 35, wherein the optical mixer comprises a90° optical hybrid (OH) comprising four output ports; wherein the stepa) comprises detecting, from the four output ports of the 90° opticalhybrid, four mixed optical signals comprising the first mixed opticalsignal, the second mixed optical signal, a third mixed optical signal,and a fourth mixed optical signal; wherein the step b) further comprisesobtaining a second differential PD signal in quadrature with the firstdifferential PD signal from the third mixed optical signal and thefourth mixed optical signal, and rectifying the second differential PDsignal to obtain a second electrical feedback signal of the at least oneelectrical feedback signals; and wherein the step c) comprises using thefirst optical phase shifter to adjust the IQ phase shift in thequadrature modulator so as to equalize the first electrical feedbacksignal and the second electrical feedback signal.